1. Field of the Invention
This invention relates to a process and apparatus for melting contaminated metalliferous scrap material. More particularly, although not exclusively, the invention relates to the melting of dirty, oily or coated aluminum scrap material.
2. Description of the Prior Art
Scrap material made of aluminum or aluminum alloy is generally recycled by melting the scrap and casting the molten metal into ingots suitable for re-use. When the scrap is contaminated with non-metallic materials, especially organic materials, it is not desirable to melt the scrap without first subjecting it to a heat treatment at a temperature below the melting point of the metal in order to drive off volatile components or to decompose gas-generating materials. If this is not done, there may be violent gas evolution during melting or excessive smoke and flame which would put heavy intermittent demands on fume control systems. Furthermore, the direct melting of contaminated thin gauge scrap in air undesirably results in large metal losses due to oxidation, e.g. as high as 15% by weight.
There are various known processes for removing the volatiles from contaminated scrap but these processes generally take place in a furnace or apparatus which is separate from the melting equipment (normally a conventional melting furnace or sidewell furnace) and thus involve a heavy capital outlay. Moreover, if the decontamination step (removal of volatiles) is carried out merely by heating the scrap in air or in atmospheres containing such gases as carbon dioxide or water vapour, a thick layer of oxide is formed on the scrap during the decontaminating process and this not only results in loss of recoverable metal when the scrap consists of thin gauge material but also causes problems during the melting step. In particular, the presence of an oxide film on the metal makes it necessary to add a large amount of a salt flux to the scrap during the melting step in order to obtain an efficient separation of the metal from the oxide surface layers, but the use of a flux means that a salt cake remains after the melting process and this poses disposal problems since such by-products are pollutants when disposed in land fill sites.
Another disadvantage of the conventional scrap decontamination methods is that the scrap must usually be separated into small pieces (if it is not already in this form) before it is subjected to the decontamination step. In order to reduce transportation costs, many forms of scrap are compressed into bales at the collection site, and so these bales have to be crushed, shredded or chopped into small pieces before they are introduced into the decontamination apparatus. Other forms of scrap generally have to be divided in the same way. This is necessary not only to ensure that all of the contaminated surfaces are exposed during the decontamination step, but also that the decontaminated product can be properly accommodated by the melting furnace. This crushing or separation step requires the provision of bulky and expensive equipment in addition to the decontamination and melting furnaces and again this results in large capital outlays.
An alternative process for continuous melting of light metal scrap is disclosed in East German Patent No. 212,054 to Primke et al (assigned to VEB Mansfeld-Kombinat Wilhelm Pieck). In this process, aluminum or other light metal scrap is melted in a plasma melting furnace employing nitrogen gas as the plasma-producing medium. In order to avoid undue reaction between the aluminum and nitrogen owing to catalysis caused by the contaminants on the metal, gaseous and dusty impurities are first removed from the scrap by suction and the scrap is fed below the surface of the metal melt to avoid direct contact between remaining impurities and the plasma arc or the furnace atmosphere above the bath. However, the apparatus required to carry out such a process is complicated and expensive because it requires a furnace with a raisable refractory chute for the feeding of the scap and a suction device for removal of impurities. The process also requires scrap which is shredded, cleaned, dried and preheated before charging which restricts and complicates the procedure.